MPEG2 (ISO/IEC 13818-2) is known as an international standard of video compression. In video compression methods such as MPEG2, in order to increase the compression efficiency by bidirectional prediction, the order of input pictures should be changed (reordering) when encoding the pictures.
When an input NTSC signal is MPEG-encoded in real time, the NTSC signal is initially converted into a digital signal, e.g., a D1 format which is a video component of a non-compressed and CCIR-601 convertible digital format, and the digital signal is encoded. At this time, the signal may fluctuate or the temporal input may deviate when using an asynchronous clock, resulting in a phenomenon in which decoded pictures are disordered.
In order to solve this problem, a process of correcting a deviation of a sync signal when inputting an analog signal, which process is generally called “time base correct (TBC)”, is required in the stage prior to an encoder.
However, as digital processing has become widespread, it is more important to suppress a deviation of a frame cycle due to asynchronicity of a clock than to suppress a signal fluctuation.
As a technique for correcting a deviation of a vertical sync signal, for example, Japanese Published Patent Application No. Hei.11-261968 discloses a video compression recording apparatus with an external input function, which is provided with a means for judging a vertical sync interval of an external input signal. In this apparatus, when the external input signal is changed by a switcher or the like and it is judged that the vertical sync interval of the external input signal is shorter than a normal vertical sync interval, a process of thinning the short-interval vertical sync signal is carried out. On the other hand, when it is judged that the vertical sync interval of the external input signal is longer than the normal vertical sync interval, capture of the external input signal is stopped for a period of time that is equivalent to the increase in the vertical sync interval. Thereby, occurrence of an incorrect stream is prevented.
Although the conventional video encoding apparatus is constructed as described above to prevent occurrence of an incorrect stream, since capture of the external signal is stopped when the vertical sync interval becomes longer than the normal interval, data are not supplied to the encoder during this period of time.
Furthermore, a buffer memory for temporarily holding data to be inputted to the encoder, having a capacity equivalent to one frame, is usually provided in the stage prior to the encoder. However, when a process of correcting a deviation of a frame cycle is to be carried out in the stage prior to the encoder, an additional memory for this process is required, which leads to an increase in device size, or another circuit should be added, resulting in an increase in costs.